update_headers.sh: arm64: Copy sve_context.h if available

The SVE KVM support for arm64 includes the additional backend
header <asm/sve_context.h> from <asm/kvm.h>.

So update this header if it is available.

To avoid creating a sudden dependency on a specific minimum kernel
version, ignore such optional headers if the source kernel tree
doesn't have them.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>